In 1860, Matsuoka traveled as part of Japan's first diplomatic mission to the United States as a surveyor aboard the Kanrin Maru along with experienced blue water sailor John Manjiro and future Meiji reformer Yukichi Fukuzawa.

[4] The deaths of Tokugawa Iemochi and Emperor Kōmei in 1866 and 1867 brought the long simmering tension between the shogunate and the imperial court to a head, sparking the Boshin War.

Bankichi would take command of the Japanese warship Banryū as forces loyal to Tokugawa Navy Admiral Enomoto Takeaki refused to surrender to the Meiji government, and escaped following the fall of the shogunate, sailing to the northern island of Ezo (now known as Hokkaido).

Believing that he could not escape due to the difference in engine power, he prepared to fight with a boarding attack, saying, "I washed my face, changed into a new shirt, and joked, today is a good day to die."

Bankichi would continue to fight valiantly against the Imperial fleet until his ammunition was exhausted, land near Benten Daiba, breaking through the enemy line, and joined the shogunate forces there.